1. Field of Disclosure
The present disclosure relates generally to plasmonic substrates and specifically to high-throughput trapping of particles on a plasmonic substrate.
2. Related Art
The ability to trap particles by light using optical tweezers generated lots of interest within the past four decades. In an optical tweezer, a tightly focused laser beam is used to create strong optical gradient forces for confining micro-particles. Because of the diffraction limit, light can only be focused down to about half the wavelength in the medium, thus setting a limit on maximum achievable optical gradient force from laser power. Since the trapping force scales as the particle radius to third power (in the quasi-static limit), and coupled with increased Brownian motion it becomes difficult to address submicron and nanoscale objects with optical tweezers. As a result, plasmonic trapping is now actively investigated to overcome the limitations of optical tweezers.
In conventional plasmonic trapping also known as plasmonic nanotweezer, the local field enhancement around a local localized surface plasmon resonance (LSPR) supporting nanostructure, generated via resonant coupling of incident photons with free electrons on metallic nanostructures, is used to achieve sub-wavelength electromagnetic field confinement. This field highly confined to the surface of the nanostructures creates strong optical gradient forces, thus offering a route for trapping submicron and nanoscale particles. Additionally, the highly localized and enhanced electromagnetic field also known as ‘optical hot spots’ can be engineered to create arbitrary optical trapping potential wells for confining particles. However, the excitation of localized surface plasmonic resonance and hence local field enhancement is also accompanied by resonant light absorption, which results in local heat generation within the volume of the plasmonic nanostructures.
In the context of plasmonic trapping, generally, the focus is on the enhanced local fields, while little attention is paid to the associated local heat generation or thermal hot spots. This local heating effect has been seen as an obstacle to stable trapping of particles on a plasmonic substrate because of heating induced thermophoresis and even boiling, which obscures the trapping process. Hence efforts have been made to suppress this heating effect such as integrating a heat sink to dissipate excess heat, and off-resonance excitation to minimize light absorption. However the emerging field of thermoplasmonics has identified the unique attribute of this heating effect in realization of nanoscale heat sources that can be remotely controlled and switched by light.
Several applications harnessing this effect and being explored include plasmonic photothermal therapy for destruction of tumor cells, photothermal imaging, and solar powered steam generation. Similar to the above-mentioned applications which rely on local heating effect enabled by resonantly excited plasmonic nanoparticles, this intrinsic heating effect could be harnessed for trapping, concentration, manipulation and sorting of micro and nanoscale particles on a plasmonic substrate. Additionally, it is important to emphasize that suppressing the heating effect leaves only the enhanced electromagnetic local field as the only photo-induced signal present, and this presents some practical challenges for plasmonic nanotweezers.
First because the enhanced electromagnetic fields or optical hot spots exist in the near-field, they produce short-range interactions. Thus, the force field due to the optical hot spots can only be felt by an object after it has diffused several nanometers close to the resonant nanostructure, where it can interact with the force field from the enhanced local field. Because the object is transported via Brownian motion, the process is inherently slow. Thus only particles sufficiently close to the resonantly excited nanostructures can be trapped in a reasonable time frame.
In addition, because the field is confined to the nanostructures, manipulating the laser source from one point to another effectively switches off the optical hot spots at the initial location and switches it on at another location. Now, if the separation between the plasmonic nanostructures (i.e. the trapping sites) is large such that near-field electromagnetic coupling is absent, then a trapped object cannot be manipulated by optical gradient forces (from near-field enhancement), as these are short-range interactions. Hence, transport of target particles over long distances, which is critical for varieties of Lab-on-a-chip application such as on-chip sorting, has not been shown using plasmonic nanotweezers. These important issues suggest that there is a need for further advancement of plasmonic nanotweezer design. These issues limit the applicability of plasmonic tweezers for various lab-on-a-chip applications such as biosensing, where the rapid delivery of analytes such as bio-particles is critical to improve plasmonic biosensor response time. As a result, efforts are now being made to address the limitations of current plasmonic nanotweezers.
It is important to emphasize that the use of a plasmonic substrate for particle trapping opens up additional applications beyond trapping of submicron and nanoscale particles by taking advantage of the high photonic density of states generated when excited at plasmonic resonance. These include biosensing (for example via LSPR resonance shifts) surface enhanced spectroscopies, as well as enhancing the radiative properties of emitters. In biosensors, for example, rapid transport, and concentration of analytes is critical for reducing the detection time as well as improving the detection limit. Hence the ability to rapidly manipulate, and sort micro and nanoparticles on a plasmonic nanostructured substrate would greatly enable several lab-on-a chip applications with plasmonic nanostructures.
However, these applications have been hampered so far because of the diffusion-limited transport of particles to the trapping sites. Rather embodiments of the present disclosure take advantage of the intrinsic heating effects from photo-induced heating of a plasmonic nanoparticle array, instead of suppressing them, to address the issue of dynamic transport of dielectric particles over long distances on plasmonic nanostructures. Embodiments of the present disclosure demonstrate rapid particle transport, high throughput concentration, dynamic manipulation, and sorting of micro and nanoscale particles on a plasmonic nanostructured substrate, by harnessing collective heating effect of arrays of plasmonic nanostructures on a substrate. Embodiments of the present disclosure synergize localized surface plasmon resonance with an optically-induced electrokinetic phenomenon known as Rapid Electrokinetic Patterning (REP).
In Rapid Electrokinetic Patterning, a tightly focused laser beam is used to heat an electrode surface made of a thin absorbing film such as ITO coating on a glass substrate. The absorbed energy dissipated into heat is transferred to the adjoining fluid medium and creates conductivity and permittivity gradients. With the application of an AC electric field, an electrothermal body force is generated in the fluid. The electrothermal force captures suspended particles in the fluid and rapidly transports them to the electrode surface. For external AC frequencies below a certain critical frequency, particles brought close to the electrode surface are captured by low frequency electrokinetic forces. Typical laser intensity used for REP is on the order of 1010 W/m2. Embodiments of the present disclosure have replaced a thin film substrate with plasmonic resonant nanostructures and for the first time harnessed the collective heating of many nanoparticles to achieve better heating efficiency at reduced laser power and focusing. Because the plasmonic nanostructures enable nanoscale heat confinement within the particles, better heat confinement is achieved with minimal thermal spreading.
The use of plasmonic nanostructured substrates presents two main advantages. First, better heating can be obtained with resonantly excited nanoparticle array due to the combined action of large absorption efficiency and collective contribution of many thermally interacting nanoparticles. This makes it possible to induce stronger electrothermal vortices for particle transport at reduced illumination intensity in comparison with use of a thin film substrate (an important factor for handling biological organisms). Second, illumination of the plasmonic nanostructures also results in creation of optical hot spots or localized surface plasmons, which could be employed for surface enhanced spectroscopies, biosensing and engineering the photonic density of states of quantum emitters such as nitrogen-vacancy centers in nanodiamonds.